Systems and methods for determining concrete strength

ABSTRACT

In one embodiment, a concrete strength testing system includes a core drill having a core barrel, a press associated with the core drill that is configured to drive the core barrel into concrete to be tested, a force sensor associated with the core drill that is configured to measure a force with which the core barrel is driven into the concrete by the press, and a depth measurement device configured to measure a depth into the concrete to which the core barrel is driven by the press.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 62/607,559, filed Dec. 19, 2017, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

Concrete is a preferred construction material for many civil engineeringstructures due to its strength, cost-effectiveness, and durability.Concrete typically comprises cement, sand, crushed rock aggregates, andwater. Variants of cementing materials and aggregates can be used toincrease performance. Most often, concrete is delivered to a projectsite as a viscous fluid in large trucks and poured into forms that maybe above or below ground. Regardless of the application, concrete isdesigned to have a specified strength. Unfortunately, concrete, and itsproperties, can be inadvertently altered by various on-site factors.These factors can include one or more of temperature, extended drivetimes or construction delays, improper mix proportions, unintended useof admixtures, incomplete/poor mixing, segregation or mixing with groundwater, or attack from chemical agents in the soil or environment.

Given that such on-site factors can adversely affect the strength ofconcrete, it is common practice to test the liquid concrete, and thecured concrete formed from it, onsite. This normally involves verifyingthe fluidity upon arrival, noting the amount of mixing the entire truckdrum has imparted into that batch, and preparing cylindrical specimensthat harden on-site and are then periodically tested for compressivestrength to confirm strength gain. Even with these safeguards,situations arise where the quality of the as-placed concrete comes intoquestion. In these events, the most robust assurance mechanism is totake a core sample of the curing concrete and test the compressivestrength. The sample size is often similar to the cylinders prepared forquality assurance when the truck arrived on-site (e.g., 4 inches indiameter). Standard specifications require that the compression testsample be twice as long as the diameter, meaning that only one datapoint can be obtained for every 8 inches of core concrete length.Additionally, the process requires that the full-length cores be cut tothe proper length with squared ends and then tested in compression.

The above-described testing process is time-consuming and only providesa discrete number of samples, which is further reduced by breaks in therecovered samples, making portions of the core unusable. It cantherefore be appreciated that it would be desirable to have alternativesystems and methods for determining the strength of the in-placeconcrete.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures. Matching reference numerals designate correspondingparts throughout the figures, which are not necessarily drawn to scale.

FIG. 1 is a front view of an embodiment of a concrete strength testingsystem.

FIG. 2 is a side view of the system of FIG. 1.

FIG. 3 is a block diagram of an embodiment of a computing device thatcan calculate concrete strength based upon data collected by the systemof FIGS. 1 and 2.

FIG. 4 is a graph that plots core strength (strength vs. depth) for atest specimen tested using a prototype concrete strength testing systemsimilar to that illustrated in FIGS. 1 and 2.

FIG. 5 is a graph that plots core strength (strength vs. depth) formultiple test specimens tested using a prototype concrete strengthtesting system similar to that illustrated in FIGS. 1 and 2.

DETAILED DESCRIPTION

As described above, it would be desirable to have alternative systemsand methods for determining the strength of the concrete. Disclosedherein are examples of such systems and methods. In some embodiments, aconcrete strength testing system comprises an instrumented core drillthat is configured to instantaneously determine the equivalentcompressive strength during drilling, thereby eliminating the need toperform compressive testing on collected cores. The drilled core holecan be much smaller (e.g., 1 inch in diameter) than the cores currentlyused in concrete strength testing, making the size of required equipmentsmaller and the influence of the coring on the structure lesssignificant. A core sample is still retrieved, however, and can be usedfor verification or calibration.

In the following disclosure, various specific embodiments are described.It is to be understood that those embodiments are exampleimplementations of the disclosed inventions and that alternativeembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure.

A goal of the disclosed inventions is to provide a system capable ofgenerating and outputting a strength profile of concrete in real time.In concept, the system is a concrete penetrometer that measures theinstantaneous concrete strength from concrete coring resistance via afully instrumented concrete coring drill motor. Part of the novelty ofthe system is the simultaneous collection of data and the computationalconversions to the in situ concrete strength profile. A significantadvantage of use of the system is that it can reveal locally weak orstrong portions of the concrete that may be missed from traditionallysized specimens.

FIGS. 1 and 2 illustrate an embodiment of a concrete strength testingsystem 10 configured to instantaneously determine concrete strength. Asshown in the figures, the system 10 includes a core drill 12. The coredrill 12 comprises an elongated core barrel 14 and can be raised andlowered using one or more presses 16, such as one or more pneumaticpresses, which are mounted to a frame 18 that includes a base 20. Insome embodiments, the core barrel 14 has a small diameter, such asapproximately 1 in. In the illustrated embodiment, there are two presses16 mounted to the base 20, one positioned on either side of the coredrill 12. Extending upward from the presses 16 are elongated piston rods22 that are connected to an upper cross-member 24 of the frame 18. Ascan be appreciated from FIG. 1, the core drill 12 is suspended from theupper cross-member 24. Positioned between the upper cross-member 24 andthe core drill 12 is a force sensor 26, such as a load cell, configuredto measure the force applied to the drill by the presses 16. Alsosuspended from the upper cross-member 24 is a depth measurement device28. In some embodiments, the depth measurement device 28 comprises astring-line displacement transducer having a string 30 that is attachedto a lower cross-member 32 of the frame 18. In such an embodiment, thedepth measurement device 28 is configured to measure the distancebetween the upper cross-member 24 and the lower cross-member 32 and,therefore, the depth to which the core drill 12, and its core barrel 14,is driven into the concrete to be tested.

With further reference to FIGS. 1 and 2, the concrete strength testingsystem 10 also includes a rotational speed measurement device 33 that isconfigured to measure the speed with which the core barrel 14 rotates.In some embodiments, the rotational speed measurement device 33comprises a rotary encoder that monitors revolutions of the core barrel14. The system 10 further comprises a pressure sensor 34, such as apressure transducer, configured to measure the pressure of fluid used toflush cuttings from the annulus around the core barrel 14. This pressureis useful to know as the fluid applies an upward (lifting) force on thecore barrel 14 during drilling, in which case the force on the coredrill 12 is actually the force applied by the press 16 minus the forceapplied by the fluid. In some embodiments, the system 10 furthercomprises a flow meter (not shown), such as a magnetic flux flow meter,which is configured to measure the flow rate of the fluid.

The concrete strength testing system 10 further includes a power meter36 that is configured to measure the power (product of the current andvoltage) drawn by the core drill 12. In other embodiments, the powermeter 36 can be replaced or supplemented with a torque measurementdevice configured to measure the torque of the motor of the core drill12. Such a measurement device can, for example, comprise one or moreload cells that measure the force required to hold the core drill 12against rotation as it is drilling the concrete or a torsional shearcell that is provided between the core drill and its core barrel 14.

In addition to the aforementioned components, the concrete strengthtesting system 10 further includes a power supply 38 (e.g., battery)that provides power to the system, a control panel 40 that can be usedby a user to operate the system, as well as a communication port 42 thatcan be used to export data collected by the system to another system,such as a desk or notebook computing device (see, e.g., FIG. 3).

A prototype system similar to that described above was developed andevaluated. The platform of the prototype system was a Milwaukee 4049,20-amp manually-operated coring machine fitted with a 1-inch innerdiameter, diamond-tip core barrel. This is a wet core drill that lowersand lifts the core barrel with a linear gear, rack-and-pinionconfiguration, wherein turning the crank controls crowd and advances orretracts the drill with a manually-applied, variable force.

Producing usable and replicable data for instantaneous strengthdetermination necessitated the isolation of variables that affect coredrilling effectiveness including: downward force on core barrel (crowd),rotational speed, torque, advancement rate, power consumption, fluidflow, and fluid pressure. Mechanically, the linear gear/crank assemblywas removed and replaced with two Parker 4MA series, 24-inch stroke,2.5-inch diameter, double-acting pneumatic cylinders. These pneumaticcylinders enable complete user control of the force applied by apneumatic press by using two air pressure regulators each thatindependently controlled the downward crowd or upward extraction force.The exact force applied to the drill motor and core barrel was monitoredusing an Omega LCCD-2K, 2000-pound capacity load cell connected betweenthe pneumatic cylinder and the core drill motor.

A Celesco SP2-50 string-line displacement transducer with a 50-inchrange was used to record the depth of coring. By recording theassociated coring time, the vertical advancement rate could also bedetermined. The rotational speed (rpm) was measured with a BEI, H20incremental rotary encoder mounted to a 2:1 ratio set of pulse wheels.Because fluid was also used to flush cuttings from the annulus aroundthe core barrel and, in turn, affects drilling performance, the pressureof the fluid was monitored with Honeywell Model AB/HP 6 psi pressuretransducer.

The variation in power that resulted from additional crowd and drillingresistance was directly monitored using a GE PQMII power quality meter.This meter combines voltage with current taken using an Omega RCT151205Acurrent coil to produce a power output that takes into account theeffects of phase shift. All data was monitored and recorded using aModel BMS16HR-53 Titan Mini-recorder computerized data acquisition unitfrom Mars Labs. The data sampling rate was 128 Hz.

The result of this instrumentation was a drilling machine with theability to provide dynamic force, velocity, pressure, power, androtational speed. In post-processing, this data could then be used todetermine the resistive force and strength of the concrete. This dataanalysis process is described below.

Preliminary verification tests were conducted using the system whereeach of the transducer outputs was checked.

Rotational Speed

Rotational speed can serve as a quality control check for the finalcalculations based on the idea that, given a constant applied drillingforce, the rate of rotation should increase or decrease withcorresponding changes in concrete strength. Rotation was calibratedthrough use of the totalizer option on the data acquisition unit. Thecore barrel was turned manually and the rotations tallied, this resultwas then compared to the total rotations recorded. Good agreement wasnoted.

Power

In lieu of measuring torque and multiply it by rpm to compute power, theelectrical power meter was implemented that simultaneously measures thecurrent, voltage, and phase angle. The importance lies in the phaseangle measurement that was not previously measured and where the actualpower draw is the product of voltage, current, and the cosine of thephase angle. Previous measurements assumed a phase angle to be small andwhere the power factor (cosine of the phase angle) was taken as aconstant of 0.9 based on spot checked values. The new power meter isthought to be a significant advantage by removing this assumption.

Displacement

While somewhat trivial compared to the other transducers, thestring-line transducer was confirmed to register the full 18 in. strokeof the pneumatic cylinder. Like rotational speed, the advancement ratewas then computed using the timestamps associated with each data point.

Pressure

Pressure of the drilling fluid, if appreciable, could reduce the netforce on the cutting edge of the core barrel. However, the anticipatedpressure range was small and the 6-psi transducer range made simplecalibration checks possible by using a simple column of water andcomparing the hydrostatic pressure with that registered. Good agreementwas noted.

Testing Procedure

The process of coring was standardized to provide baseline measurementsof crowd, displacement, rpm, and power prior to making contact with theconcrete surface. The base plate of the core rig was equipped with slotsto allow for the installation of a mechanical rebar splice as a means tosecure the machine to the shaft surface. The testing procedure wasperformed as follows:

-   -   1. Set data acquisition unit to scan.    -   2. Balance transducers.    -   3. Turn on water.    -   4. Power core drill.    -   5. Wait 5 seconds.    -   6. Begin recording data.    -   7. Wait 3 seconds.    -   8. Slowly turn the vent knob from the “OFF” to the “DRILL”        position allowing the core barrel to come in contact with the        concrete surface gently.    -   9. Turn the Pressure knob from the “OFF” to the “DRILL”        position.    -   10. Monitor transducer readings during drilling operations.    -   11. Once drilling operations are complete allow the drill to run        for an additional 3 seconds.    -   12. Turn off power.    -   13. Stop recording data.    -   14. Carefully extract core barrel and check for any concrete        prior to setting up at the next drilling location.        Results

The system was instrumented to provide all necessary information used tocalculate the specific energy. This, in turn, is correlated to thestrength of rock or cemented materials during rotary, non-percussivedrilling operations. Therein, crowd, rotational speed, torque, anddisplacement measurements are required (Equation (1)):

$\begin{matrix}{e = {\frac{F}{A} + {\frac{2\pi}{A}\left( \frac{NT}{u} \right)}}} & (1)\end{matrix}$wherein:

-   -   e=specific energy (psi)    -   F=crowd (lbs)    -   A=core bit area (in²)    -   N=rotational speed (rev/min)    -   T=torque (lb-in)    -   u=penetration rRate (in/min)        The concrete penetrometer used the same equation but where        power, P, was measured directly (Equation 2):        P=TN2π  (2)        By combining Equations (1) and (2), Teale's expression        simplifies into Equation (3):

$\begin{matrix}{e = {\frac{F}{A} + \frac{P}{Au}}} & (3)\end{matrix}$The specific energy is then equated to compressive strength (f′c) usingan empirical relationship (Equation 4) where the coefficients a and bare both a function of the penetration rate:

$\begin{matrix}{{f^{\prime}c} = \frac{b + {\sqrt{\;}\left( {b^{2} - {4{ae}}} \right)}}{2a}} & (4)\end{matrix}$wherein f′c=compressive strength (psi).

The strength is then averaged per 1/16″ and graphed against depth.Notably, all of the above calculations can be automatically performed bya computing device that receives the data collected by the dataacquisition unit. FIG. 3 shows an example configuration of such acomputing device. The computing device 50 generally comprises aprocessing device 52, memory 54, a user interface 56, and one or moreinput/output (I/O) devices 58, each of which is connected to a systembus 60. The processing device 52 can, for example, include a centralprocessing unit (CPU) that is capable of executing computer-executableinstructions stored within the memory 54. The memory 54 can include anyone of or a combination of volatile memory elements (e.g., RAM, flash,etc.) and nonvolatile memory elements (e.g., hard disk, ROM, etc.). Theuser interface 56 can comprise one or more devices that can enter userinputs into the computing device 50, such as a keyboard and mouse, aswell as one or more devices that can convey information to the user,such as a display. The I/O devices 68 can comprise components thatenable the computing device 50 to communicate with other devices, suchas a network adapter.

The memory 54 (a non-transitory computer-readable medium) storessoftware applications (programs) including an operating system 62 and aconcrete strength calculation program 64 configured to calculateconcrete strength and, optionally, graph the results. The concretestrength calculation program 74 includes computer-executableinstructions, which may be comprised by one or more algorithms (i.e.,computer logic), which can be executed by the processing device 52.Results computed by the program 74 can optionally be stored in adatabase 66.

FIG. 4 shows the results from a sample coring. FIG. 5 shows the resultsof several cores taken on a single shaft. The variation in averagestrength is consistent with the calculated strength in disparate regionson the specimen.

This device was used to test concrete samples too small to show trendsthat were needed to demonstrate whether or not concrete was beingcompromised by submerged casting environments. Without the novel device,strength variation throughout the concrete specimens could not have beendetermined. To date, 35 of the 58 test specimens have been cored in asimilar manner. In order to analyze the effect of slurry on concrete inthe cover region, the average strength was determined for each core. Thestrengths associated with the cores in the annular cover region werethen compared to the average interior strength, which was used as abaseline for each shaft. These results are grouped by castingenvironment (slurry type) and shown in Tables 1-3 below.

TABLE 1 Strength ratios - Bentonite Bentonite Baseline Shaft ViscosityStrength Ratio Ratio To Ratio To # (sec) (psi) To Crease Cover 1 Cover 29 57 5136.75 0.81 0.74 0.96 10 90 4654.67 0.85 0.84 0.88 15 56 4712.351.05 1.05 1.12 Average 0.92

TABLE 2 Strength ratios - Polymer Polymer Baseline Shaft ViscosityStrength Ratio Ratio To Ratio To # (sec) (psi) To Crease Cover 1 Cover 211 65 4180.95 1.05 1.24 1.07 17 85 4345.25 0.91 0.91 0.93 19 63 5736.000.84 0.86 0.97 Average 0.98

TABLE 3 Strength ratios - water Water Baseline Shaft Viscosity StrengthRatio Ratio To Ratio To # (sec) (psi) To Crease Cover 1 Cover 2 6 264383.24 0.92 0.95 0.93

When looking at the loss of strength between the interior and coverregion cores, the bentonite cast shafts show the largest reduction withan average ratio of 0.92. The largest individual loss of 0.74 was alsoin a bentonite cast shaft. The polymer cast shafts stayed fairlyconsistent as did the water. Again, previous methods of testing thesespecimens would have resulted in only 6 data points (3 inside the cageand 3 outside the cage). The new device/method was able to fulldiscriminate along the full core length.

The invention claimed is:
 1. A method for testing concrete strength, themethod comprising: drilling into concrete to be tested with a coredrill; measuring a force with which a core barrel of the core drill isdriven into the concrete; measuring a rotational speed of the corebarrel; measuring a current, voltage, and phase angle of the core drill;measuring a pressure of fluid used to remove cuttings from around thecore drill that are produced while drilling the concrete; measuring adepth to which the core barrel is driven into the concrete; andcalculating the strength of the concrete based upon the measured force,rotational speed, current, voltage, phase angle, pressure, and depth. 2.The method of claim 1, wherein the drilling into the concrete comprisesdriving the core barrel into the concrete using a pneumatic press. 3.The method of claim 1, wherein the measuring of the force comprisesmeasuring the force with a load cell associated with the core drill. 4.The method of claim 1, wherein the measuring of the depth comprisesmeasuring the depth using a string-line displacement transducer.
 5. Themethod of claim 1, wherein the measuring of the rotational speed of thecore barrel comprises measuring the rotational speed using a rotaryencoder.
 6. The method of claim 1, wherein the measuring of the pressureof a fluid comprises measuring the pressure using a pressure transducer.7. The method of claim 1, wherein the measuring of the current, voltage,and phase angle comprises measuring the current, voltage, and phaseangle using a power meter.
 8. The method of claim 1, wherein thecalculating of the strength of the concrete comprises calculating theconcrete strength using a computer configured to compute the concretestrength based upon the measured force, rotational speed, current,voltage, phase angle, pressure, and depth.